1. Field of the Invention
The present invention relates to a charging device for a secondary battery installable in electronic equipment such as mobile phones and portable computers.
2. Discussion of the Background
Charging devices that charge secondary batteries, such as on-board Li+/Li polymer batteries installable in electronic equipment such as mobile phones and portable computers, are often included in the electronic equipment itself.
When the charging the secondary batteries, it is necessary to charge rapidly while inhibiting heat generation in the electronic equipment.
For example, when a linear controlling type charging technique is used, although heat generation in the equipment can be prevented, the charging cannot not be executed in a short time.
By contrast, when step-down direct-current to direct-current (DC/DC) charges or pulse charging techniques are used, charging can be executed rapidly while inhibiting the electronic equipment from generating heat.
However, although charging by step-down DC/DC or pulse controlling is effective, such effects cannot be obtained when the secondary battery is in poor condition (depleted).
More specifically, because internal impedance of the battery is increased when the battery is depleted, in usual pulse wide modulation (PWM) control ripple voltage of the secondary battery becomes larger, the battery is depleted earlier, and power supply rejection ratio (PSRR) of the target to which electricity is supplied is not secured, which are problems.
Furthermore, if an attempt is made to solve these problems by just decreasing the duty cycle, the supplied power becomes insufficient, lengthening the time required for charging.
In order to solve the problem described above, Japanese Patent No. 3,420,325 proposes a charging device that performs duty-cycle decreasing and current adjustment in accordance with battery condition, and therefore the ripple voltage can be alleviated. Accordingly, depletion of the secondary battery can be prevented, and PSRR to the supplied object can be secured. However, the problem prolonged charging time when the battery is depleted remains unsolved.
Accordingly, there is a need for a technology to effectively charge the depleted battery in the charging device without prolongation of the charging time.